1. Field of the Invention
The present invention relates to an optical material and an optical system using it and, more particularly, to a low refractive index high-dispersion optical material, for example, applicable to optical elements such as lenses, filters, mirrors, refracting optical elements, diffracting optical elements, and so on, and an optical system of optical equipment such as cameras, binoculars, microscopes, and so on, using the optical material.
2. Related Background Art
As one of conventional approaches to correction of chromatic aberration of the optical system composed of only refracting optics, there is a method of combining glass materials of different dispersion characteristics. For example, in the case of objectives of telescopes etc., axial chromatic aberration is corrected by combining a positive lens of a low-dispersion glass material with a negative lens of a high-dispersion glass material. For this reason, there were cases where chromatic aberration was not corrected adequately if the composition or the number of lenses was limited or if available glass materials were limited.
For the purpose of correction of this chromatic aberration, there are suggestions on methods of controlling the refractive index and Abbe""s number in order to expand the range of the optical constants of glass materials, thereby obtaining the low-refraction high-dispersion glass materials, for example, in Japanese Patent Application Laid-Open No. 6-32631, Japanese Patent Application Laid-Open No. 61-9262, Japanese Patent Publication No. 4-33740, and so on.
On the other hand, other references, e.g. SPIE Vol. 1354 International Lens Design Conference (1990) etc., disclose methods of reducing the chromatic aberration by use of a diffracting optical element provided with a diffraction grating having diffracting action in part of a lens surface or an optical system, in contrast to the methods of reducing the chromatic aberration by the combination of glass materials.
These methods make use of the physical phenomenon in which a refracting surface and a diffracting surface in the optical system demonstrate opposite directions of occurrence of chromatic aberration against rays of a certain reference wavelength.
Further, such diffracting optical element can be provided with an aspherical-lens-like effect by changing periods of the periodic structure of the diffraction grating and is thus greatly effective to reduction of aberration.
Comparing them herein as to refraction of rays, a ray appears as one ray even after refracted by a lens surface, whereas a ray appears as a plurality of rays of plural orders after diffracted by a diffractive surface.
When the diffracting optical element is used as a lens system, it is thus necessary to determine the grating Structure so as to concentrate light of the used wavelength range into specific orders (which will also be referred to hereinafter as xe2x80x9cdesigned ordersxe2x80x9d) by adequately enhancing the diffraction efficiency of rays of the designed orders. When light is concentrated in the specific orders, the intensity of rays of diffracted light of the other orders is low. When the intensity is zero, diffracted light thereof appears null. If there exist rays of diffraction orders other then the designed orders, they will be focused at positions different from those of the rays of the designed orders and appear as flare light.
The layouts capable of reducing this decrease of diffraction efficiency are presented in Japanese Patent Application Laid-Open No. 9-127321 and Japanese Patent Application Laid-Open No. 11-44808. The layouts realize high diffraction efficiency in a wide wavelength range by optimizing dispersion properties and thicknesses of gratings of materials different from each other.
Specifically, they disclose the diffracting optical elements wherein a plurality of optical materials (layers) are stacked on a substrate and wherein the diffraction grating is formed in the step shape, kinoform, or binary shape resulting from approximation thereof to the step shape, or the like in at least one of interfaces between the mutually different optical materials.
In each of the diffracting optical elements described in Japanese Patent Application Laid-Open No. 09-127321 and Japanese Patent Application Laid-Open No. 11-44808, the combination of a relatively high-refraction low-dispersion material with a relatively low-refraction high-dispersion material is used in order to obtain the layout having the high diffraction efficiency in the wide wavelength range. Specifically, the materials used are as follows: BMS81 (nd=1.64, xcexdd=60.1: available from Ohara Inc.) and the plastic optical material PC (nd=1.58, xcexdd=30.5: available from Teijin Chemicals Ltd.) in the case of Japanese Patent Application Laid-Open No. 9-127321; COO1 (nd=1.5250, xcexdd=50.8: available from Dainippon Ink and Chemicals, Inc.), the plastic optical material PC (nd=1.58, xcexdd=30.5: available from Teijin Chemicals Ltd.), BMS81 (nd=1.64, xcexdd=60.1: available from Ohara Inc.) etc. In the case of Japanese Patent Application Laid-Open No. 11-44808.
For further enhancing the optical performance of the aforementioned diffracting optical elements, we investigated the optical materials commercially available, or the optical materials under research and development and obtained the distribution as illustrated in FIG. 1.
The materials of the stack diffracting optical elements described in the Japanese Patent Applications Laid-Open No. 9-127321 and Laid-Open No. 11-44808 also fall in the distribution of FIG. 1.
As for the materials suggested in the aforementioned Japanese Patent Applications Laid-Open No. 6-32631 and Laid-Open No. 61-9262 and Japanese Patent [Publication No. 4-33740] Publication Laid-Open No. 6-16450, their refractive indexes and refractive-index dispersions are (nd=1.585 to 1.660, xcexdd=40.5 to 32.5), (nd=1.5945 to 1.6925, xcexdd=27.3 to 36.6), and (nd=1.55 to 1.65, xcexdd=27 to 35), respectively, and these materials also fall in the distribution of the existing substances of FIG. 1.
It is seen that 2-ethozy-ethyl methacrylate (nd=1.483, xcexdd=32) seems only one material with low refractive index and high dispersion and that there have been few such materials developed.
An object of the present invention is to provide an optical material with low refractive index and high dispersion, which was not known heretofore, and an optical system using it.
A first aspect of the present invention is an optical material which is a mixture of materials comprising a first material having a refractive index of not more than 1.45 for the d-line and a second material having an Abbe""s number, indicating wavelength dispersion in the visible region,.of not more than 25, wherein a relation between the refractive index for the d-line (nd) and the Abbe""s number (xcexdd) indicating the wavelength dispersion in the visible region is defined as follows:
ndxe2x89xa6xe2x88x926.667xc3x9710xe2x88x923xcexdd+1.70.
A second aspect of the invention is the optical material of the first aspect wherein said Abbe""s number (xcexdd) indicating the wavelength dispersion in the visible region is less than 40.
A third aspect of the invention is the optical material of the first aspect or the second aspect wherein said second material comprises particles having the grain size in the range of 2 to 100 nm.
A fourth aspect of the invention is the optical material of the first aspect, the second aspect, or the third aspect wherein said first material is an amorphous fluororesin.
A fifth aspect of the invention is the optical material of either one of the first to fourth aspects wherein said second material is particles of a composite metal oxide of titanium and silicon (Sixxe2x80x94Ti(1xe2x88x92x)O2 having the Abbe""s number (xcexdd) of 24.4.
A sixth aspect of the invention is the optical material of either one of the first to fifth aspects wherein said first material is an amorphous fluororesin, said second material is particles of a composite metal oxide of titanium and silicon (Sixxe2x80x94Ti(1xe2x88x92x)O2 having the Abbe""s number (xcexdd) of 24.4, and a weight ratio of the particles and said amorphous fluororesin is in the range of 45:100 to 75:100.
A seventh aspect of the invention is the optical material of the first aspect, the second aspect, or the inthird aspect wherein said first material is a dimethylsilicone resin.
An eighth aspect of the invention is the optical material of either one of the first to third aspects and the seventh aspect wherein said second material comprises particles of titanium oxide (TiO2).
A ninth aspect of the invention is the optical material of the first aspect, the second aspect, or the third aspect wherein said first material is a dimethylsilicone resin, said second material is particles of titanium oxide (TiO2), and a weight ratio of the titanium oxide and said dimethylsilicone resin is in the range of 18:100 to 70:100.
A tenth aspect of the invention is an optical material wherein a relation between a refractive index for the d-line (nd) and an Abbe""s number (xcexdd) indicating wavelength dispersion in the visible region satisfies the following condition:
ndxe2x89xa6xe2x88x920.01xcexdd+1.70.
An eleventh aspect of the invention is an optical material which is a mixture of materials comprising a first material having a refractive index of not more than 1.40 for the d-line and a second material having an Abbe""s number, indicating wavelength dispersion in the visible region, of not more than 15, wherein a relation between the refractive index for the d-line (nd) and the Abbe""s number (xcexdd) indicating the wavelength dispersion in the visible region is defined as follows:
ndxe2x89xa6xe2x88x920.01xcexdd+1.70.
A twelfth aspect of the invention is the optical material of the tenth aspect or the eleventh aspect wherein said Abbe""s number (xcexdd) indicates the wavelength dispersion in the visible region is not more than 40.
A thirteenth aspect of the invention is the optical material of the eleventh aspect wherein said second material comprises particles having the grain size in the range of 2 to 100 nm.
A fourteenth aspect of the invention is the optical material of the eleventh aspect or the thirteenth aspect wherein said first material comprises an amorphous fluororesin.
A fifteenth aspect of the invention is the optical material of the eleventh aspect, the thirteenth aspect, or the fourteenth aspect wherein said second material comprises particles of titanium oxide (TiO2).
A sixteenth aspect of the invention is the optical material of the eleventh aspect, the thirteenth aspect, the fourteenth aspect, or fifteenth aspect wherein said first material is an amorphous fluororesin, said second material is particles of titanium oxide (TiO2), and a weight ratio of the titanium oxide and said amorphous fluororesin is in the range of 7:100 to 90:100.
A seventeenth aspect of the invention is an optical material which is a mixture of materials comprising a first material having a refractive index for the d-line in the range of 1.45 to 1.55 both inclusive and a second material having an Abbe""s number, indicating wavelength dispersion in the visible region, of not more than 10, wherein a relation between the refractive index for the d-line (nd) and the Abbe""s number (xcexdd) is defined as follows:
ndxe2x89xa6xe2x88x926.667xc3x9710xe2x88x923xcexdd+1.70.
An eighteenth aspect of the invention is the optical material of the seventeenth aspect wherein said Abbe""s number (xcexdd) indicates the wavelength dispersion in the visible region is not more than 40.
A nineteenth aspect of the invention is the optical material of the seventeenth aspect or the eighteenth aspect wherein said second material comprises particles having the grain size in the range of 2 to 100 nm.
A twentieth aspect of the invention is the optical material of the seventeenth aspect, the eighteenth aspect or the nineteenth aspect wherein said second material is ITO (indium-tin-oxide).
A twenty-first aspect of the invention is the optical material of either one of the seventeenth to twentieth aspects wherein said first material is polymethyl methacrylate.
A twenty-second aspect of the invention is the optical material of either one of the seventeenth to twenty-first aspects wherein said first material is polymethyl methacrylate, said second material is particles of ITO (indium-tin-oxide), and a weight ratio of the particles and said polymethyl methacrylate is in the range of 30:100 to 250:100.
A twenty-third aspect of the invention is the optical material of either one of the seventeenth to twentieth aspects wherein said first material is an amorphous polyolefin.
A twenty-fourth aspect of the invention is the optical material of either one of the seventeenth to twentieth aspects wherein said first material is an amorphous polyolefin, said second material is particles of ITO (indium-tin-oxide), and a weight ratio of the particles and said amorphous polyolefin is in the range of 44:100 to 150:100.
A twenty-fifth aspect of the invention is the optical material of either one of the seventeenth to twentieth aspects wherein said first material is a copolymer of methyl methacrylate and styrene.
A twenty-sixth aspect of the invention is the optical material of either one of the seventeenth to twentieth aspects wherein said first material is a copolymer resin of methyl methacrylate and styrene, said second material is particles of ITO (indium-tin-oxide), and a weight ratio of the particles and said copolymer resin is in the range of 43:100 to 140:100.
A twenty-seventh aspect of invention is an optical member comprising the optical material as set forth in either one of the first aspect to the twenty-sixth aspect.
A twenty-eighth aspect of the invention is an optical system comprising the optical member of the twenty-seventh aspect.
A twenty-ninth aspect of the invention is a diffracting optical element using the optical material as set forth in either one of the first aspect to the twenty-sixth aspect.
A thirtieth aspect of the invention is an optical system comprising the diffracting optical element of the twenty-ninth aspect.
A thirty-first aspect of the invention is an optical device comprising the optical system of the twenty-eighth aspect or the thirtieth aspect.
A thirty-second aspect of the invention is a method for producing an optical material, comprising a step of decreasing a filling factor of a first material, and a step of filling gaps of the first material of the decreased filling factor with a second material having an Abbe""s number different from that of the first material, thereby producing an optical material having a desired refractive index and Abbe""s number.
A thirty-third aspect of the invention is an optical member comprising the material produced by the production method as set forth in the thirty-second aspect.
A thirty-fourth aspect of the invention is an optical system comprising the optical member of the thirty-third aspect.
A thirty-fifth aspect of the invention is the optical system of the thirty-fourth aspect wherein said optical member is a diffracting optical element.
A thirty-sixth aspect of the invention is an optical device comprising the optical system of the thirty-fourth aspect or the thirty-fifth aspect.